The Stability Of Nanobubbles And The Measurement Of Their Internal Density

Nanobubbles are nanoscopic gas bubbles at the liquid-solid interface or in liquid solution,which were first proposed to explain the steps in the measured forces between hydrophobic surfaces in water.Later,these bubbles were directly observed by atomic force microscopy(AFM)on different substrates.Their existence has been confirmed by degassing experiments,as well as many other techniques.Nanobubbles are strange and quite different from the conventional bubbles.The morphology obtained by AFM shows that nanobubbles are spherical cap shaped,the radii of curvature is in the order 100 nm to 1000 nm,due to the Laplace pressure arises from curved interface and the resulting increases in gas solubility,these bubbles should dissolve in solution rapidly(less than tens milliseconds)according to classical diffusion theory.However,experiments show that nanobubbles are stable for hours.Besides,the contact angle of nanobubbles is generally 150° to 170° on hydrophobic surface,which is much larger than the contact angle of macroscopic bubbles on the same substrate.The early researches were focused on these two questions as reviewed in some references.Detlef Lohse proposed that pinning of the three phase line is crucial for stability of nanobubbles,the Laplace pressure is balanced by gas oversaturation in the liquid.More recently,a second stage in the physics of nanobubbles has evolved,because of the availability of new experimental and theoretical tools.These new results brought about some important changes in our viewpoints,(1)Some reports have found that the life time of nanobubbles in degassed water is much longer than that predicted by theory.In this case,the bubbles are thermodynamically unstable;there must be some kinetic effects to hamper the diffusion flow from the bubbles.(2)Bulk nanobubbles can exist for hours even they are also thermodynamically unstable.And they are also a polydisperse colloid system.The diffusional interaction plays an important role for the evolution of the system,which received little attention in previous study.(3)Nanoscale resolved spectroscopic techniques have been used to collect structure information from individual nanobbble,e.g.,soft X ray imaging,wet transmission electron microscopy and force spectroscopy using AFM.These results can help us understand how properties transition from macroscopic bubbles to nanoscopic counterparts bubbles.The aim of this research is theoretically analyze and explain these new phenomena.We start from the interfacial bubbles and focus on the differences between the nanobubbles and their macroscopic counterparts.The free path of molecular collisions inside nanobubbles is larger than the size of bubbles.Therefore,we used the free-molecule flow model to calculate the internal state of the bubble.Second,we considered the influence of the long-range interaction between the gas-liquid interface and the solid on the Laplace pressure.We found that the influence of these two factors were small.We further considered the stability of nanobubbles in a confined solution in soft X-rays imaging and transmission electron microscopy.Because the volume of the entire system was constrained,the Laplace pressure could be cancelled out by the tensile stress of the liquid.Therefore,they had different stable characteristics.All the previous discussions are in the framework of classical gas theory are not sufficient to explain the stability of nanobubbles.For this reason,we can only treat the gas inside the bubbles as a “black box”,assuming that the inside of the bubbles is a homogeneous material.The two most important parameters affecting its dissolution kinetics are density and solubility.If the density inside the bubble is comparable to the density of the condensed matter,we can explain the stability of the bubble in the degassed solution.Using the force spectroscopy,we measured the gas density at the gas-solid interface whose density was indeed much greater than the density of the gas at the corresponding Laplace pressure.However,the high density state inside the bubble was undetermined in this work.In the outlook,we proposed two methods to verify the internal high density of nanobubbles.One was to measure the response of nanobubbles to external pressure on an atomic force microscope;the other is to measure the evolution of average size of nanobubbles over time.However,at nanometer scale,as long as the internal gas molecules satisfy the ergodic conditions in statistical mechanics and the interaction between the molecules is weak,the equation of state of the gas is determined accordingly.Therefore,explaining the origin of the high density is still very difficult.